The present invention relates in general to a roof structure, and more particular, to such a roof structure constructed of tension members and compression members arranged in the form of a cable truss dome suitable for spanning delineated areas of various size for providing a protective roof thereover for stadiums, arenas, and the like, where a dome like volume has been created.
In recent years, dome and roof structures have been constructed of light weight membranes in order to achieve lower cost and to improve their performance. These structures have been constructed of either synclastic surfaces held-up and stiffened by air pressure or anticlastic surfaces formed by ridged arches, masts, and/or cables against which the membrane has been prestressed. the synclastic surface may have a low aspect ratio, that is, the ratio of surface area to delineated plan area. The resulting low aspect ratio has an important advantage for permanent membrane roofs, such as low profile air structures, since the membrane cost is as much as 70% of the roof cost, the balance being cables, clamps, and compression ring. However, the disadvantage of the air structure has been its dependence on mechanical systems for keeping the roof up and the resulting requirement for providing air tight buildings, snow melting systems, emergency generators, revolving doors, and the like. On the other hand, the anticlastic surface has a very high aspect ratio resulting in higher roof costs per unit of delineated plan area. For example, those anticlastic surfaces employing masts and cables have high aspect ratios in order to provide sufficiently low membrane stress, while those utilizing arches have high aspect ratios due to design considerations such as arch buckling.
In addition to the foregoing problems encountered with regard to the use of light weight membranes consisting of either synclastic or anticlastic surfaces, there is a problem of roof insulation. Previous attempts to incorporate insulation in air supported roofs have required the installation of snow melting systems separate and apart from the interior heating system so as to prevent condensation of moisture on the upper covering membrane thereby impairing the insulation value of the insulation. In addition, there is the requirement to pressurize the insulation space to pressures higher than the interior pressure of the enclosed delineated area in order to prevent the collapse of the insulation.
Another difficulty encountered with certain roof structures of the prior art is the inability to resist wind induced up-lift forces exceeding the dead weight of the roof structures. Further, difficulties are often encountered in erecting these structures constructed of continuous flexible elements such as cables with discontinuous stiff elements such as compression struts as they generally do not have the required stiffness except in their complete assemblied condition.
It can therefore be appreciated that there is an unsolved need for a tension structural system, such as a cable truss dome, which is adaptable for spanning large areas where the tension elements form low shallow arches that support a light weight membrane on their upper ridge cables and held down by valley cables arranged between the arches, and which is adaptable for providing a low aspect ratio tension structural system which eliminates the need for supporting air pressure and allows for the insulation of low cost light weight membrane roofs.